Under normal conditions, most T lymphocytes are made in the thymus. However, the thymus degenerates with age, beginning around puberty. Because output of new T cells from the thymus is proportional to its mass, age-related thymic atrophy results in a progressive decline in production of naive T cells. In the peripheral lymphoid system, decreased thymic output is compensated by homeostatic expansion of existing T cells. While this avoids frank T cell lymphopenia, the result is a T cell pool with an increasingly oligoclonal repertoire, rather than the broad spectrum of immunity conferred by newly generated, naive thymic T cells. Thus, aging is associated with accumulation of T cell immunodeficiencies (or immunoinsufficiencies), resulting in increased susceptibility to infectious disease, decreased vaccine response, decreased anti- tumor surveillance, increased autoimmunity, and other disorders. Decreased capacity to make new T cells is also a substantial limitation in hematopoietic stem cell transplantation, which is an established therapy for diseases like leukemia, and an emerging therapy for autoimmune disorders like lupus erythematosus and multiple sclerosis. Correcting/preventing age-related thymic degeneration (atrophy) is thus of substantial importance for enhancing quality of life, and decreasing health care costs in adults and the elderly. Notably, the thymus can be induced to completely regrow, but the most efficient means for this (surgical castration) is impractical. Nonetheless, this plasticity shows the potential for devising practical means for inducing regeneration. The aims of this project are to use recently devised, robust physical methods (laser microdissection, microarray) and computational modeling to create accurate global lists of thymic stromal genes in their native state, both in the atrophied thymus, and during various regrowth phases (initiation,log, peak) induced by castration. Stromal gene expression signatures will then be analyzed to reveal changes that occur in atrophy, and during the regrowth response. Informatic and biological validations will be used to identify key regulators in these processes, which will be followed-up by conventional biological approaches. In addition to an in-depth understanding of thymic stromal biology and the regrowth process, we expect to reveal potential targets for therapeutic approaches for thymic regeneration.